A respiration mode APRV (Airway Pressure Release Ventilation), which, similarly to the CPAP (Continuous Positive Airway Pressure) mode, is a mode with a continuous positive airway pressure, is known from practice. Contrary to the CPAP respiration mode, an upper pressure Phigh is applied in the APRV mode. This pressure Phigh is regularly lowered to a lower pressure level Plow for a short duration each, and this pressure reduction is called “pressure release.” The patient being respirated by means of APRV can breathe spontaneously at any time, but the pressure release is not synchronized with the patient's breathing activity. It is typical of the APRV mode that a duration Thigh, during which the pressure Phigh is applied, is usually longer than a duration Tlow, during which the lower pressure Plow is present. FIGS. 1a and 1b show an example of a respiration in the APRV mode in a simulation.
During respiration in the APRV mode, the mean respiration pressure is maintained at a comparatively high level, which leads to improved oxygenation. The removal of CO2 is supported by the pressure release. Due to the short duration Tlow and the short duration of pressure release, ventilation of the lungs to such an extent that alveoli would collapse and could no longer participate in the ventilation shall be prevented. The duration Tlow is set such that complete expiration is prevented from occurring. As can be recognized from FIG. 1, the pressure release is terminated before the patient flow has risen to 0 L/minute (see the times t=5.5 sec; t=13 sec; t=21.5 sec in FIG. 1).
A new set-point, which is circumscribed by “optimal flow termination based on a percentage of peak expiratory flow,” is proposed for APRV respiration in the patent application US 2006/0174884 A1 of Nader M. Habashi. This set-point will hereinafter be called % PEF (Peak Expiratory Flow). The parameter % PEF is set as a percentage of a peak expiratory flow PEF. The pressure release is terminated when the instantaneous expiratory flow of the patient relative to the peak expiratory flow has dropped below the percentage set as % PEF (cf. FIG. 2). As a result, the algorithm automatically adapts the duration of the pressure release to changes in the lungs. According to the above-mentioned US 2006/0174884 A1, the end of duration Tlow shall be selected at the moment at which % PEF, which may be in a setting range between 25% and 50% of PEF (Peak Expiratory Flow), has reached or equals the set % PEF. The expiratory gas flow is monitored for this purpose, the PEF is determined, and the pressure release phase is terminated when the expiratory gas flow drops below the preset percentage % PEF.
One drawback of both a manual setting of Tlow and of an automatic adaptation of Tlow to a preset percentage of the peak expiratory flow is, however, as was described above, that this value does not represent the actual end-expiratory lung volume or the volume actually expired, but only a point in time at which it is assumed based on the reduced expiratory flow that the patient has expired to a certain extent. It is therefore not possible to evaluate whether, for example, dorsal areas of the lungs, which are under the effect of hydrostatic pressure, are effectively prevented by the termination of expiration from collapsing. In addition, in case of an existing inhomogeneity of the lung disease over the organ, there still may be lung areas that collapse during the expiration or during the pressure release phase even during expiration that is terminated already at a high percentage of the peak expiratory flow, as this is described in Habashi's US patent application. This is disadvantageous for the patient.